Colored glass is produced by adding very small amounts of coloring substances to the vitrifiable mixture. In particular, coloring mixtures are added to the transparent glass matrix during the glass melting step by means of dispensers into which the colorant is loaded, usually in the form of granules or tablets.
The colorants used in this sector are mainly metal oxides, in particular oxides of transition metals.
For example, iron oxide (III) is used in order to impart an olive green color to the glass under reducing conditions or else a blue-green color under oxidizing conditions.
In order to produce dark (black) glass, wide use is made of coloring compositions based on nickel oxide.
In particular, nickel oxide added in low amounts (usually at a concentration of about 1%) to a mixture of oxides of transition metals (manganese dioxide, chromium oxide and cobalt oxide) is added to the vitrifiable mixture in order to obtain a dark glass characterized by a color with tones ranging from blue to violet, to black.
The dark glass, essentially black, is greatly used in the production of food grade containers or containers for cosmetic and pharmaceutical use in order to protect the contents (such as food and/or drugs) from radiation having a wavelength of less than 500 nanometers (nm), in particular from ultraviolet radiation.
The use of coloring compositions containing nickel oxide renders the glassmaking process and the procedure for producing the coloring mixture itself very hazardous to the health of workers.
It is well known and also specified in the European Community tri classification that nickel is an element suspected of being a cause of cancer. In particular, prolonged or repeated exposure to nickel-based, compounds such as nickel oxide may provoke serious damage to various organs or strong allergic reactions on the skin.
For this reason, the use of nickel, or compounds containing it, is strictly regulated. In particular, the release of nickel is limited under the European Community REACH regulation N. 1907/2006, Annex XVII.
The use of nickel oxide on an industrial level entails a very lengthy and costly authorization procedure. Therefore, at present, the processes for producing nickel oxide-based glass colorants and the glass coloring processes that use these types of colorants involve a series of very restrictive rules that are inconvenient to follow.
For this reason, in the sector there is a greatly felt need to have a composition for coloring glass that is free of nickel oxide and can assure a dark color of the glass which is comparable to that obtained using the colorants presently available for this purpose.
The opportunity of having at one's disposition a nickel-free coloring composition for obtaining dark glass would above all avoid the costly and demanding bureaucratic procedures required for the use of a hazardous substance, such as nickel oxide.
Moreover, the use of a nickel-free coloring composition would enable the process for producing colored glass to be streamlined, since the expedients and/or operations tied to the use of a colorant containing nickel oxide would no longer be necessary.
For example, it would no longer be necessary to seal the equipment, just as authorization from the European Community would no longer be mandatory.